The present invention relates to a male or female threaded tubular element of a threaded tubular connection which is particularly able to resist both static and cyclic stresses.
The present invention also relates to a threaded tubular connection which is particularly suitable for resisting both static and cyclic stresses.
Threaded tubular connections comprise a male threaded element at the end of a first pipe and a female threaded element at the end of a second pipe which may be a great length pipe or a coupling. Such threaded connections are used in particular to constitute casing strings or production strings or drillpipe strings for hydrocarbon wells or for similar wells such as for example geothermal wells.
In its API specification 5B, the American Petroleum Institute (API) defines threaded connections between casing pipes or between production pipes in particular with tapered threadings with trapezoidal or round triangular threads.
Other types of threaded connections are also known which use straight or tapered two-step threads: see, for example, U.S. Pat. No. 4,521,042.
Until recently, casing pipes or production pipes had essentially to be capable of resisting different combinations of static stresses (tension, axial compression, plane bending, internal or external pressure) despite their limited thickness resulting from the need to be able to exploit a deep well and insert a variety of columns of different diameters one into another.
In contrast, drillpipes, which are only used to drill wells, are subjected to substantial cyclic stresses but are not subjected to size limitations, since a single string of a given diameter is downhole at a given time.
If not strictly limited, cyclic stresses lead during operation to fatigue ruptures which start at the root of the threads, generally on the side of the load flanks which are under load.
This preferred location for initiation of fatigue cracks results from a stress concentration at the junction between the load flank and the thread root.
To improve the resistance to cyclic stresses, it is necessary to reduce the maximum level of the stresses by reducing the general level of stresses on the load flank and by producing the least possible angularity of the junction between the load flank and the thread root.
API specification 7D defines drillpipes with robust tapered threads which are adapted to operational stresses. API specification 7D threads are triangular in shape and very rounded with load and stabbing flanks which are each disposed at 30xc2x0 with respect to the normal to the axis of the threaded element.
The load flank is that which is disposed on each thread on the side opposite the free end of the element. This definition will be used throughout the present document.
The thread root is rounded in an arc of a circle with a radius of 0.97 mm (0.038xe2x80x3) centred on the axis of the thread root; this arc of a circle joins tangentially with the flanks.
The angle of 60xc2x0 between the thread flanks resulting from the triangular shape of the threads enables the radius of the arc of a circle to be substantial.
The thread crests are truncated so as to avoid any radial interference between the thread crests and the thread roots of the mated threading.
The height of the truncated threads is 3.08 mm (0.121xe2x80x3) which corresponds to twice the height of API 5B threaded connections.
These means can, however, prove to be insufficient since U.S. Pat. No. 4,549,754 describes a threading profile which is modified with respect to API specification 7D for drillpipes which renders it capable of further reducing stress concentration.
The thread of U.S. Pat. No. 4,549,754 shows in cross section a root which is not symmetrical but comprises a rounded zone the centre of which is offset towards the stabbing flank (opposite the load flank) and with the radius increased by about 50% with respect to the API radius, namely 1.45 mm (0.057xe2x80x3).
This rounded zone joins the load flank tangentially while it joins with the stabbing flank in a less critical profile: a simple straight segment or a radius of 0.81 mm (0.032xe2x80x3) followed by a straight segment.
The thread root is thus more undercut than an API thread and hence requires a large starting pipe thickness in order to cut the threads.
Such a disposition cannot be envisaged for strings of pipes for exploiting wells since these are subjected to both static and dynamic stresses.
Such demands on stress resistance are now being encountered in underwater strings connecting the sea bed to offshore hydrocarbon exploitation platforms.
Such columns of pipes, known to the English-speaking skilled persons as xe2x80x9crisersxe2x80x9d, are subjected to cyclic stresses caused in particular by currents which induce vibrations in the column, by waves, by tides and by possible displacement of the platforms themselves.
Such demands on stress resistance are also encountered in onshore wells, in particular when dropping rotating pipes in order to cement wells in the very frequent case of wells which deviate from the vertical and have bends.
For this reason, improvements to threaded tubular connections for casing pipes, for production pipes or for xe2x80x9crisersxe2x80x9d have been sought in order to increase their fatigue strength.
Patent application WO 98/50 720 describes such an improved threaded tubular connection.
The threadings described in that document have trapezoidal threads derived from the threads of API specification 5B known as xe2x80x9cbuttressxe2x80x9d threads.
The trapezoidal shape of the threads limits the risk of deformation of the threaded elements which can lead to them dislodging during coupling, in particular by overtorquing.
The thread roots are substantially rectilinear and join to each of the flanks via a rounded zone the radius of which is comprised between 10% and 50% of the total width of the thread root (and preferably between 16% and 26% of that total width), the rounded zone terminating tangentially to the flank and to the thread root.
The thread heights are such that radial interference between the root of one thread and the crest of the corresponding thread of the mated threading is completely avoided by maintaining a radial clearance of at least 0.25 mm (0.010xe2x80x3).
Taking into account the threadings given in the example, the radii at the thread root are of the order of 0.5 mm as opposed to 0.15 mm for the radii specified in API 5B.
Such radii may appear low if they are compared with those of drillpipes but the trapezoidal form of the threads used does not allow as large radii to be formed as in the case of triangular threads unless it is acceptable to drastically reduce the bearing surface of the contacting flanks.
The threadings of document WO 98/50720 are also not adapted to interfering type threads which have a radial interference between the thread crests of one threading and the corresponding thread roots of the mated threading. The threads shown are xe2x80x9cwedgexe2x80x9d type threads with a variable width, like those shown in U.S. Re. Pat. No. 30,647.
The aim of the present invention is to produce a male or female threaded tubular element for threaded tubular connections, which are particularly resistant both to:
a) static stresses, in particular axial tension, axial compression, bending, torsion, internal or external pressure, dislodging during connection, either simple or combined (for example tension+internal pressure);
b) cyclic stresses.
In the remainder of the present document, such a threaded element will be described as having an anti-fatigue profile.
The present invention also aims to ensure that the threaded tubular element of the invention can be formed with all types of threadings: tapered, straight, straight-tapered combinations, with one or more steps, with trapezoidal or triangular threads, which may be interfering or non-interfering; non interfering threadings are, for example, of the type described in European patent application EP 0 454 147 with simultaneous contact of the two flanks with those of the mated thread (also called xe2x80x9crugged threadxe2x80x9d), with an axial interference fit, or of the wedge type with a varying width as described, for example, in U.S. Re. Pat. No. 30,647.
A further aim is that the threaded element can be produced and inspected easily.
The threaded element of the invention must be able to be used to constitute threaded connections for strings of hydrocarbon production pipes, for well casings or for underwater exploitation (xe2x80x9crisersxe2x80x9d) or for similar uses.
A still further aim is to produce threaded tubular connections which are sealed, in particular gas tight, even under cyclic stresses.
In a variation, the threaded element of the invention must be able to be used in drillpipe strings.
A still further aim is to produce a threaded tubular connection in which only one of the threaded elements, for example the female element, has been modified to resist cyclic stresses but which is compatible with a non modified mated threaded element.
In a variation, both threaded elements of the threaded tubular connection have been modified to resist cyclic stresses.
In accordance with the invention, the male or female threaded tubular element with an anti-fatigue profile is formed at the end of a pipe and comprises an external male or internal female threading depending on whether the threaded element is male or female in type.
The threads comprise a thread crest, a thread root, a rectilinear load flank, a rectilinear stabbing flank and two tangential junction zones termed xe2x80x9cthread rootxe2x80x9d zones.
Each of the two tangential thread root junction zones is disposed between the thread root and one of the two thread flanks, termed the xe2x80x9ccorresponding flankxe2x80x9d, and comprises an arc of a circle.
At least one of the two tangential thread root junction zones, termed the xe2x80x9cmultiple radius zonexe2x80x9d, comprises an arc of a circle termed the xe2x80x9cprincipal arcxe2x80x9d, wherein the support circle cuts the support straight line of the corresponding flank at a point termed the xe2x80x9cflank reference pointxe2x80x9d, and also a regular curve termed the xe2x80x9csecondary curvexe2x80x9d either side of the principal arc which tangentially joins this to the corresponding flank and to the thread root: a non tangential junction would introduce a stress peak which is particularly deleterious to fatigue at the singular junction point.
Similarly, the second curve must be regular, i.e., must not have a singular point which could introduce a stress peak at this location.
At the flank reference point, the tangent to the support circle of the principal arc makes a strictly positive acute angle with the support straight line of the corresponding flank.
In the remainder of the document, the positive sense is such that the principal arc does not undercut the material of the threads: a negative angle between the tangent and flank would clearly be particularly deleterious for the fatigue behaviour.
Said support circle of the principal arc cuts or is tangential to the support straight line of the thread root and the tangent to said support circle makes, at the point of intersection or at the tangential point under consideration, an angle comprised between xe2x88x9215xc2x0 and +15xc2x0 with the support straight line of the thread root.
When the circle support of the principal arc is tangential to the support straight line of the thread root, this angle is zero and the secondary curve on the thread root side reduces to a point.
When the thread root reduces to a point, by convention the support straight line of the thread root is the straight line passing through the thread root which is parallel to the axis of the threaded element.
The shape and disposition of the principal arc in any multiple radius zone is perfectly defined:
by the position of the flank reference point;
by the angle between the tangent to the circle support of the principal arc and the corresponding flank;
and by the angle between the tangent to said circle and the thread root.
The radius of the principal arc of each multiple radius zone of the thread root is larger than that of the arc of the circle termed the xe2x80x9cstandard arcxe2x80x9d passing through said flank reference point which would by itself constitute a tangential junction zone between the corresponding flank and the thread root.
The invention can thus use a high junction radius in the critical zones located towards the middle of the junction zone where the principal arc is located and lower radii at the junction with the corresponding flank and with the thread root where the secondary curves are located, without consuming too much of the thread height.
For a given thread height, the closer the flank reference point to the thread root, the more flank surface is available to bear on the corresponding surface of the mated threaded element, increasing the static performance of the resulting threaded connection.
In the case of prior art threaded elements, the radial height of the junction zone (distance of the flank reference point from the thread root) is proportional to the radius of this zone. As a result, for these threaded elements and for a given thread height, any gain in the fatigue characteristics (cyclic stresses) results in weakening the static characteristics.
In the case of the present invention, because of the positive angle between the tangent to the support circle of the principal arc and the flank, the radial height of the junction zone is proportional to the radius of the principal arc but the coefficient of proportionality is all the more lower as this positive angle increases. Thus either the fatigue behaviour under static conditions is improved or the static conditions for a given fatigue behaviour or the fatigue behaviour and the static characteristics are improved simultaneously.
Preferably, the angle between the tangent to the support circle of the principal arc of the multiple radius zone under consideration and the corresponding flank at the flank reference point is in the range +10xc2x0 to (70xc2x0xe2x88x92J), J designates the corresponding flank angle, i.e., the angle between the rectilinear portion of the flank under consideration and the normal to the axis of the threaded tubular element. The flank angle is said to be positive when the flank under consideration tends not to overhang the thread root.
Highly preferably, the angle between the tangent to the support circle of the principal arc of the multiple radius zone under consideration and the load flank at the flank reference point is in the range +15xc2x0 to (45xc2x0xe2x88x92J), J having the same meaning as above.
A configuration with a positive or zero flank angle is preferable from the point of view of concentrating stresses at the thread roots.
Preferably, the radius of the principal arc of the multiple radius zone is in the range 150% to 250% of that of the standard arc which would constitute a tangential junction zone passing via the flank reference point.
Preferably again, each secondary curve of the multiple radius zone is an arc of a circle.
Highly preferably, the ratio of the radius of the arc of each secondary curve to that of the principal arc is in the range 0.1 to 0.4.
The minimum value of this ratio avoids an excessive increase in stresses at the secondary curves.
The maximum value of this ratio limits the overall extent of the multiple radius zone.
The invention can be applied by modifying the profile of the threads either on the side of a single flank, in particular the load flank which is generally the most loaded, or on the two flanks.
It can also be applied both to triangular and to trapezoidal threads with a fixed or varied width and for tapered, straight, combined, simple or multiple-step threadings.
A variety of embodiments will be described below, in a non-limiting way, which illustrate the scope of the invention.
The invention also provides a threaded tubular connection with a high resistance to static or cyclic stresses, comprising a male threaded tubular element at the end of a first pipe connected by screwing to a female threaded tubular element at the end of a second pipe by means of a male threading on the male threaded tubular element and a female threading on the female threaded tubular element.
The term xe2x80x9cpipexe2x80x9d means both a great length pipe and a short pipe such as a coupling.
The threads of each of the threadings comprise a thread crest, a thread root, a rectilinear load flank, a rectilinear stabbing flank and four junction zones each comprising an arc of a circle.
Of these four zones, two zones, termed tangential thread root junction zones, each join the thread root to a flank termed the corresponding flank and two zones, termed thread crest junction zones, each join the thread crest to a flank.
The profile and disposition of each thread crest junction zone are adapted so as not to interfere with the tangential thread root junction zone of the mated threaded element.
At least one of the two threaded elements, male and female, is a threaded tubular element with an anti-fatigue profile of the present invention.
Preferably in a variation, at least one thread crest junction zone of a threaded tubular element opposite a tangential thread root junction zone with multiple radii of a mated threaded tubular element with an anti-fatigue profile is a zone termed a follower which comprises two arcs of a circle which join tangentially to each other namely a principal arc and a secondary arc, this latter producing the tangential thread crest junction to the corresponding flank.
Further, at the point termed the xe2x80x9chigh junctionxe2x80x9d of the corresponding flank where the support circle of the principal arc of the following zone cuts the support straight line of the corresponding flank, the tangent to said circle makes a strictly negative acute angle with the support straight line of the flank under consideration.
Under the convention indicated above, such a sign means that the principal arc of the thread crest bites into the material of the thread.
Such a disposition enables the surface area of the flanks in contact to be increased for a given thread height.
In an advantageous variation from the cost viewpoint, only one of the threaded elements, male or female, is of the anti-fatigue type of the invention and is compatible with the other threaded element which is a prior art threaded element.
In an advantageous variation from the viewpoint of maximising performance, the two threaded elements, male and female, are of the anti-fatigue type of the invention.
In a variation, the threaded tubular connection of the invention is applicable to interference type threadings in which the thread crest of one threading radially interferes with the thread root of the mated threading.
In a further variation, the threaded tubular connection of the invention is applicable to threadings in which the two flanks of each thread are in contact, with or without contact pressure, with the two flanks of the mated threading thread, over at least a portion of the length of the threading: the invention is thus applicable to threads known as xe2x80x9crugged threadsxe2x80x9d with an axial interference fit or with a wedge of varying height.
Other advantages and characteristics of the invention will become clear from the detailed description below and from the accompanying drawings, which not only serve to clarify comprehension of the invention but also contribute to the definition thereof, as appropriate.